https://gizmodo.com/what-if-planet-nine-is-a-bowling-ball-size-black-hole-1838527880

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“Primordial black holes” are a class of proposed objects that formed as a result of the chaotic early days of the universe. Like any other black hole, they would be incredibly dense regions where gravity warps space so much that light cannot escape. But these would weigh far less than stars, since they weren’t formed out of stars like the black holes we’ve actually observed—they would have formed out of places of leftover extra density in the rapidly expanding early universe.

The article and the people that presented the theory aren't taking it too seriously, but supposedly it's not entirely off the table.